Paper No. 290-3
Presentation Time: 1:40 PM


WANG, Shidi, Department of Geology, Northwest University, Xian, 710069, China, HELLER, Paul L., Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071,, JONES, Nick, Enhanced Oil Recovery Institute, University of Wyoming, University of Wyoming, Laramie, 82071, and FAN, Majie, Department of Earth and Environmental Sciences, University of Texas at Arlington, Arlington, TX 76019
Paleoaltimetry of Rocky Mountain uplifts have been determined from stable isotope analyses of carbonate materials in Laramide basins. Such estimates require additional proxies to evaluate. Here we use flexural analysis of the Lance (Maastrichtian) and Fort Union (Paleocene) formations in the Powder River and Wind River basins of Wyoming to evaluate the verisimilitude of paleoaltimetry, plate rigidity and structural history of Laramide foreland.

Our approach involves comparing basin cross-sectional profiles with elastic flexural model results utilizing nearby thrust loads. Adjustable parameters include load height, plate rigidity and basin fill density. Best-fit results indicate that topographic relief by the developing southern Bighorn Mts and Casper Arch reached c. 1.5 km during Lance deposition and an extra 2 km of topographic load was added during Fort Union time. These values are confirmed by subsidence in the eastern Wind River Basin. In addition, erosional unroofing, as evidence by clastic compositions in the basin fill, suggests an additional 3.2 km of rock uplift took place during this period of time resulting in c. 6.7 km of net structural relief by the end of Paleocene time. This structural relief is similar to other estimates. Cumulative relief of 3.5 km between the Bighorn Mts and the Powder River basin is similar to that determined from stable-isotope paleoaltimetry for the end of Ft Union time (4.5+1.5 km), validating the method.

More surprising is the range in effective elastic thickness (EET) suggested by the best-fit models. While the Powder River basin suggests an EET of 40-55 km, only a 10-20 km EET is needed to explain the Wind River basin (WRB) data. This suggests that the mechanism of subsidence of the WRB might be very different than the simple infinite-elastic plate model that has been generally applied to the Laramide foreland. Such a mechanism would help explain the large subsidence but short wavelength subsidence of some Wyoming basins.